Method for vertical and lateral control of iii-n polarity

ABSTRACT

Disclosed herein is a method of: depositing a patterned mask layer on an N-polar GaN epitaxial layer of a sapphire, silicon, or silicon carbide substrate; depositing an AlN inversion layer on the open areas; removing any remaining mask; and depositing a III-N epitaxial layer to simultaneously produce N-polar material and III-polar material. Also disclosed herein is: depositing an AlN inversion layer on an N-polar bulk III-N substrate and depositing a III-N epitaxial layer to produce III-polar material. Also disclosed herein is: depositing an inversion layer on a III-polar bulk III-N substrate and depositing a III-N epitaxial layer to produce N-polar material. Also disclosed herein is a composition having: a bulk III-N substrate; an inversion layer on portions of the substrate; and a III-N epitaxial layer on the inversion layer. The III-N epitaxial layer is of the opposite polarity of the surface of the substrate.

This application is a divisional application of U.S. Pat. No. 9,396,941,issued on Jul. 19, 2016, which claims the benefit of U.S. ProvisionalApplication No. 61/383,869, filed on Sep. 17, 2010. The provisionalapplication and all other publications and patent documents referred tothroughout this nonprovisional application are incorporated herein byreference.

TECHNICAL FIELD

The present disclosure is generally related to control of III-Npolarity.

DESCRIPTION OF RELATED ART

Gallium nitride (GaN), a high performance semiconductor for both opticaland electronic devices, can be grown in the c-direction of its latticewith two different polar faces, nitrogen-(N—) or gallium-(Ga—) polar.The face, or polar orientation, of the material establishes many of itsproperties, from chemical reactivity and dopant incorporation tospontaneous and piezoelectric-induced electric field directions in thecrystal (Stutzmann et al., Phys. Status Solidi B 228 (2001) 505).Control of the polarization fields and, thus, polarization induceddoping is the basis of Ga-polar and N-polar GaN-based high electronmobility transistor operation. On heterogeneous substrates, such assilicon carbide and sapphire (Al₂O₃), the substrate orientation and/orgrowth conditions, doping levels, and buffer or nucleation layerproperties are used to control the polarity of resulting GaN epilayers(Stutzmann et al., Phys. Status Solidi B 228 (2001) 505; Brown et al.,J. Appl. Phys. 104 (2008) 024301; Sun et al., Appl. Phys. Lett. 93(2008) 131912; Liu et al., Appl. Phys. Lett. 91 (2007) 203115).

On sapphire substrates, polarity control has been extended to createlateral polarity heterostructures (LPHs). In these structures N- andGa-polar regions are fabricated on the same sapphire substrate throughsubtractive patterning of nucleation layers which result in Ga-polarmaterial where the nucleation layers are present and N-polar materialwhere they are not (Stutzmann et al., Phys. Status Solidi B 228 (2001)505; Mita et al., J. Cryst. Growth 311 (2009) 3044; Katayama et al.,Appl. Phys. Lett. 89 (2006) 231910; Yang et al., J. Appl. Phys. 94(2003) 5720). Efficient current transport across the polarity interfacehas been demonstrated, leading to devices such as n/n junctions, p/njunctions, and even MESFETs (Mita et al., J. Cryst. Growth 311 (2009)3044; Aleksov et al., Appl. Phys. Lett. 89 (2006) 052117; Collazo etal., Appl. Phys. Lett. 91 (2007) 212103; Collazo et al., Phys. StatusSolidi A 207 (2010) 45). However, the requirement to use sapphire wafersreduces the use of these materials, as lattice and thermal mismatchbetween substrate and epitaxy introduce dislocations as well asrestricting the possible thickness of the material.

In order to develop LPHs on GaN substrates, homoepitaxial polaritycontrol must be demonstrated. Current techniques for polarity inversionon GaN rely on Mg-induced inversion. In the case of heavily Mg-dopedp-type layers, spontaneous polarity inversion has been demonstrated onGaN homoepilayers, switching the doped layer from Ga-polar to N-polar(Pezzagna et al., J. Cryst. Growth 269 (2004) 249; Kamler et al., J.Cryst. Growth 282 (2005) 45; Ramachandran et al., Appl. Phys. Lett. 75(1999) 808). This approach leads to uncontrolled inversion domainboundaries and often results in dopant clustering within the film,impacting film quality and resultant device performance (Van de Walle etal., J. Cryst. Growth 189/190 (1998) 505; Hansen et al., Appl. Phys.Lett. 80 (2002) 2469). Furthermore, the interface between the N-andGa-polar material is highly faceted (Romano et al., Appl. Phys. Lett. 77(2000) 2479). Closely related alternatives, monolayers of Mg as well asthin Mg_(x)N_(y) layers, have also been used to convert GaN polarityfrom Ga- to N-polar. Although these alternative approaches do not sufferfrom the dopant clustering issue, the interface between the N-andGa-polar material is still highly faceted (Grandjean et al., J. Cryst.Growth 251 (2003) 460).

BRIEF SUMMARY

Disclosed herein is a method comprising: providing a sapphire, silicon,or silicon carbide substrate having an epitaxial layer of N-polar GaN;depositing a mask layer on the epitaxial layer; removing a pattern fromthe mask layer to produce open areas and masked areas; depositing an AlNinversion layer on the open areas; removing any remaining mask layerfrom the substrate; and depositing a III-N epitaxial layer on theN-polar GaN epitaxial layer and the AlN layer to simultaneously produceN-polar material on the N-polar GaN epitaxial layer and III-polarmaterial on the AlN layer.

Also disclosed herein is a method comprising: providing an N-polar bulkIII-N substrate; depositing an AlN inversion layer on the substrate;depositing a III-N epitaxial layer on the AlN inversion layer to produceIII-polar material on the AlN inversion layer.

Also disclosed herein is a method comprising: providing a III-polar bulkIII-N substrate or a sapphire substrate having an epitaxial layer ofIII-polar III-N material; depositing an inversion layer on thesubstrate; and depositing a III-N epitaxial layer on the inversion layerto produce N-polar material on the inversion layer.

Also disclosed herein is a composition comprising: a bulk III-Nsubstrate; an inversion layer on portions of an N-polar or III-polarsurface of the substrate; and a III-N epitaxial layer on the inversionlayer. The III-N epitaxial layer is of the opposite polarity of thesurface of the substrate.

Also disclosed herein is a method comprising: providing a substratehaving an surface layer of an N-polar or III-polar III-N material;depositing an inversion layer on the surface layer in a pattern leavingexposed regions of the surface layer; depositing a first III-N epitaxiallayer on a portion of the inversion layer; and depositing a second III-Nepitaxial layer on a portion of the exposed regions. The first III-Nepitaxial layer is of the opposite polarity of the surface layer, andthe second III-N epitaxial layer is of the same polarity as the surfacelayer. The first III-N epitaxial layer, the second III-N epitaxiallayer, or both comprise a region surrounded by an edge that is not incontact with the other of the first III-N epitaxial layer or the secondIII-N epitaxial layer.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention will be readily obtainedby reference to the following Description of the Example Embodiments andthe accompanying drawings.

FIG. 1 shows a schematic of the two main steps in achieving lateral andvertical crystal polarity control. Possibilities of possible iterationsshown to the right.

FIG. 2 shows a schematic of patterned growth process on sapphiresubstrates.

FIG. 3 shows a schematic of the process using homoepitaxy on Ga-polarsurfaces. The left side shows subtractive patterning, while the rightshows a lift-off method

FIGS. 4A and 4B show SEM images of alternating polarity GaN onMOCVD-grown N-polar GaN templates on sapphire. FIG. 4A: Overall viewshowing a generally sharp interface between each polarity. FIG. 4B: Viewof growth over a hillock, showing the difference between themorphologies on Ga- and N-polar growth over the hillocks.

FIGS. 5A and 5B show SEM images of lateral polarity growth directly onsapphire. FIG. 5A: Overall view showing flat surfaces and sharpinterfaces. FIG. 5B: View showing the difference between field growth,with rough N-polar material, and confined Ga-polarity growth.

FIG. 6 shows a cross-sectional SEM showing the intersection of N- andGa-polar stripes. Black double-headed arrow denotes the lateralinversion boundary, and the white double-headed arrow highlights thevertical inversion.

FIGS. 7A and 7B show ECCI images of (FIG. 7A) Ga-polar region, withthreading screw dislocation (TSD) density 1.1×10⁹ cm⁻², and (FIG. 7B)N-polar region with TSD density of 8.8×10⁸ cm⁻². Examples of threadingscrew/mixed dislocations are shown with arrows, threading edgedislocations are circled, and step edges are highlighted by dottedlines.

FIG. 8 shows a cross-sectional TEM of the vertical and lateral polarityinversion edge. Examples of IDBs contained in the N-polar heteroepilayerare shown with black arrows, while the IDB between the intentional N-and Ga-polar regions is shown with a white arrow.

FIG. 9 shows an SEM image of lateral polarity structure on HVPE N-polarGaN substrate. Faceted features inside the N-polar region and raggednature of interface are believed to arise from misalignment of thelithographic mask defining the regions with the substratecrystallographic directions.

FIGS. 10A and 10B show ECCI images of alternating polarity growth on aHVPE substrate. FIG. 10A: N-polar region with dislocation density of1×10⁷ cm⁻² (consistent with substrate). FIG. 10B: Ga-polar region withhigher dislocation density (2×10⁹ cm⁻²). Arrows show the location ofthreading screw/mixed dislocations.

FIGS. 11A and 11B show schematics of some of the devices possible withthe disclosed technique. FIG. 11A: Wavelength converter, FIG. 11B:waveguide, where the voids are left after etching out N-polar material.

FIG. 12 shows a cross-sectional SEM of a sample made by HVPE growth on aMOCVD grown patterned template.

FIG. 13 shows a cross-sectional TEM of the interface between thesubstrate, MOCVD-grown template, and HVPE regrowth. Image shows theinversion domain boundary between regions of Ga-polar and N-polarmaterial.

FIG. 14 shows a SEM of the surface of the patterned polarity inversionon a Ga-polar substrate. This shows fully coalesced N-polar materialover the ALD Al₂O₃ inversion layer.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

In the following description, for purposes of explanation and notlimitation, specific details are set forth in order to provide athorough understanding of the present disclosure. However, it will beapparent to one skilled in the art that the present subject matter maybe practiced in other embodiments that depart from these specificdetails. In other instances, detailed descriptions of well-known methodsand devices are omitted so as to not obscure the present disclosure withunnecessary detail.

Disclosed herein is a method to control local GaN polarity on GaNsubstrates without necessarily using a Mg-induced inversion. This methodrests on the development of thin alternative inversion layers and theuse of the confined epitaxial growth technique to pattern andselectively grow the inversion layer. Used in conjunction with optimizedsubsequent GaN growth processes, the technique has resulted instructures demonstrating both vertical and lateral polarity inversion onGaN substrates. Scanning electron microscopy shows a smooth interfacebetween regions of different polarities and a smooth surface, the resultof equivalent growth rates for both polarities. Electron channelingcontrast imaging is used to determine the dislocation density of eachregion. Transmission electron microscopy (TEM) images show full polarityconversion of the inverted layer. This process enables a wide range ofvariable polarity devices.

The method may be used to create patterned regions of +c face (0001,“gallium polar”) material and −c face (0001, “nitrogen polar”) materiallaterally and vertically on III-N and other substrates. A III-N materialis a compound of nitrogen and one or more group III elements. The methodmay also be used to implement regions of crystallographically invertedIII-N materials that are periodically arrayed for optical applications,novel electronics devices, and quasi-phase matched material fornon-linear optics.

The general outline to this approach can be described in two main steps:creating a patterned inversion layer followed by the simultaneous growthof inverted III-N on the inversion layer and continuing polarity on theexposed III-N substrate surface. The general view of this method isshown in FIG. 1. However, the first step of creating and patterning theinversion layer can differ depending on the substrate. The interlayerpatterning process may be practiced in at least three areas:heteroepitaxy on sapphire, homoepitaxial efforts on −c face substrates,and homoepitaxy on +c face substrates. Although the examples given hereare for GaN on sapphire and native-GaN substrates, the inversion layersderived could be used for other substrates as well as ternary andquaternary III-nitrides.

As used herein, any reference to a layer or portions thereof may referto the entirety of such layer or portions thereof, or to a non-zerofraction of such layer or portions thereof. As used herein “entiresubstrate” may refer to the working area of the substrate. For example,the outer edges of the surface of the substrate in some cases might notbe processed and are not included in the “entire substrate”. An N-polarIII-N material has a −c face (as that term is understood in the art) asthe top or exposed surface of the material. A III-polar III-N materialhas a +c face (as that term is understood in the art) as the top orexposed surface of the material.

On sapphire substrates, GaN grows N-polar on the bare substrate, butwith a thin AlN layer, inverts to Ga-polar. Most patterning in this wayis done by growing epilayers of AlN and GaN, then patterning throughsubtractive etching to reveal the substrate. In the disclosed method,the first step involves deposition of a mask material on the substrate,which is generally silicon dioxide or silicon nitride, but could be anymaterial that will withstand the subsequent steps of the method, such asgrowth temperatures of up to 1200° C. Lithography with a lift-off layermay be used to pattern the areas for the mask, deposit the maskmaterial, and then lift-off the remaining resist. Afterwards, thepatterned sample is loaded into a growth reactor and exposed toconditions resulting in selective growth of the inversion layer (AlNbetween 150 and 500 Å). The sample is unloaded, and the mask is removed.For oxide and nitride masks, this involves immersion in HF-basedsolutions. The outline of this process, including subsequent growths, isillustrated in FIG. 2.

Although this step can be accomplished by etching through an AlNepilayer or other types of epilayers, using confined growth to producethe inversion layer results in a higher quality epitaxial material,setting the stage for higher quality subsequent material. Also, by usinglift-off and selective epitaxial growth instead of etching, a pristine,“epiready” substrate surface is retained, also setting the stage forbetter subsequent growths.

This embodiment may be performed by: providing a sapphire substratehaving an epitaxial layer of N-polar GaN; depositing a mask layer on theepitaxial layer; removing a pattern from the mask layer to produce openareas and masked areas; depositing an AlN inversion layer on the openareas; removing any remaining mask layer from the substrate; anddepositing a III-N epitaxial layer on the N-polar GaN epitaxial layerand the AlN layer to simultaneously produce N-polar material on theN-polar GaN epitaxial layer and III-polar material on the AlN layer. Themask layer by be deposited by: depositing a layer of lift-off materialon the N-polar GaN epitaxial layer; lithographically processing thelift-off material to produce the open areas and the masked areas; anddepositing a mask material on the entire substrate. Removing the patternmay be by: removing any remaining lift-off material and any maskmaterial disposed thereon from the substrate. Any mask layers, lift-offmaterials and techniques, and lithographic techniques that arecompatible with the method may be used.

In homoepitaxial growths, the polarity of the substrate generallycarries into the film. For N-polar (−c face) substrates a thin, lowtemperature AlN (e.g. 100-500 Å grown at 550-750° C.) film can functionas an inversion layer. This may be grown at a high V/III ratio(10,000-30,000), and other ranges are possible. Surface preparation inthe form of cleaning steps or chemical etching may be performed prior toinversion layer growth to aid the growth of the inverted polaritylayers. A KOH-etching step (e.g. 4M KOH, 25-50° C., 10-20 minutes) maybe added prior to patterning or after mask deposition and prior toinversion layer growth. Roughening the substrate on a nano-scale may aidin the growth of the inverted layer. Otherwise, the method schematicfollows that in FIG. 2. However, the material quality improvement fromthe confined growth and preservation of the substrate surface are evenmore pronounced.

This embodiment may be performed by: providing an N-polar bulk III-Nsubstrate; depositing an AlN inversion layer on the substrate;depositing a III-N epitaxial layer on the AlN inversion layer to produceIII-polar material on the AlN inversion layer. The inversion layer maybe deposited in a pattern using any patterning technique compatible withdepositing AlN on a III-N substrate. Then, depositing the III-Nepitaxial layer can simultaneously produce III-polar material on the AlNinversion layer and N-polar material on the exposed portions of theN-polar III-N substrate.

In the case of +c face substrates, by adding an inversion layer thatresembles single crystalline sapphire, −c face material can be created.Al₂O₃ deposited by atomic layer deposition (ALD) may be used. In thiscase, the patterning steps are shorter than in the previous twoembodiments, as a mask during inversion layer growth is not required.Instead the inversion layer is patterned either directly followed bywet-etching, or using lift-off patterning and oxide deposition. Thedirect patterning approach is illustrated in FIG. 3, while the lift-offpatterning is similar to that in FIG. 2, but with resist instead of agrowth mask. These inversion layers may be thin (100-500 Å), able towithstand high temperatures (900-1200° C.) under growth conditions(ammonia and hydrogen atmosphere), and must exhibit semi-crystallinestructure. The as-deposited ALD oxide is amorphous, so it requirestemperature treatment to achieve a semi-crystalline nature. This isaccomplished during the ramp and nitridation step in the simultaneousdual-polarity growth process. Since ALD oxide deposition is notselective, the improvement in the quality of selectively growth ofinversion layers is not applicable here. However, using a liftoffprocess still provides the best surface for subsequent epitaxial growth.

This embodiment may be performed by: providing a III-polar bulk III-Nsubstrate; depositing an Al₂O₃ inversion layer on the substrate; anddepositing a III-N epitaxial layer on the Al₂O₃ inversion layer toproduce N-polar material on the Al₂O₃ inversion layer. The inversionlayer may be deposited in a pattern using any patterning techniquecompatible with depositing Al₂O₃ on a III-N substrate. Then depositingthe III-N epitaxial layer can simultaneously produce N-polar material onthe Al₂O₃ inversion layer and III-polar material on the exposed portionsof the III-polar III-N substrate.

In another embodiment, laterally separated regions of N-polar andIII-polar material are formed. A III-N layer is deposited on a portionof an inversion layer. The portion may be part of the inversion layer orall of the inversion layer. A second layer of III-N material is grown ona portion of the exposed regions of the substrate having no inversionlayer. Again, this portion may be part of the exposed regions or all ofthe exposed regions. The two layers may be coplanar, are of oppositepolarity, and are patterned such that at least one of the layers has aregion that does not contact the other layer. In one simple example,there may be an exposed stripe across the substrate with N-polarmaterial on one side of the stripe and III-polar material on the otherside. The patterns may be designed to form any number of such islands ofone polarity not touching the other polarity. Any of the substrates,inversion layers, III-N materials, patterning methods, and growthtechniques disclosed herein may be used. The first and second epitaxiallayers may be grown in two separate growth processes or simultaneously.

The III-N layer may be deposited by any method for growing a III-Nmaterial. Thicker layers may be produced by using hydride vapor phaseepitaxy, a technique that is known in the art. A combination of growthmethods may also be used to produce thick layers. Any bulk III-Nsubstrate may be used including, but not limited to, a GaN substrate. Inany of the methods, the III-N epitaxial layer may be any III-N materialincluding binary, ternary, and quarternary compounds such as, but notlimited to GaN, InGaN, AlGaN, and AlInGaN.

Once the specific inversion layer is patterned, the sample is reloadedin the chamber under conditions that result in the simultaneous growthof + and −c face material. The samples may be nitridated (NH₃ flowrate >0.5 slm) at high temperatures (1000-1200° C.) for 10-30 minutes tofacilitate N-polar growth. Alternatively, nitridation can occur at evenlower flow rates over a longer time. For inversion layers of ALD Al₂O₃,this step at high temperature aids in preparing the inversion layer forepitaxial growth, as it increases the crystallinity of the material.After nitridation, a low temperature GaN buffer layer may be grown(550-800° C., 50-500 Å) before annealing at growth temperature(900-1100° C.) for 15-30 minutes. Higher temperature buffer layers couldbe used to improve crystalline quality. Annealing and subsequent growthmay be carried out under an atmosphere of ammonia, hydrogen, andnitrogen. The introduction of nitrogen aids in smoothing the −c facegrowth as well as equalizing the growth rates of the two polarities.Further improvement could be made by removing hydrogen from the process,using nitrogen as the carrier gas. The epilayer growth parameters mayinclude growth temperature between 1000-1100° C., V/III ratios between2000 and 5000, and gas flows of 1-2 slm NH₃, μmol TMG, 1-2 slm N₂, and1.5-2.5 slm H₂ as a carrier gas. Other source gases such as TEG may beused, and flow rates may be optimized for individual reactors.

The method may produce a composition comprising: a bulk III-N substrate;an inversion layer on portions of an N-polar or III-polar surface of thesubstrate; and a III-N epitaxial layer on the inversion layer. The III-Nepitaxial layer is of the opposite polarity of the surface of thesubstrate. The epitaxial layer may be, for example, 0.2-50 μm thick,including 5 μm. The III-N epitaxial layer may extend to cover portionsof the substrate not having the inversion layer thereon and thereby havethe same polarity as the surface of the substrate.

Vertical and lateral polarity switching on GaN HVPE substrates, sapphiresubstrates, and GaN epilayers has been demonstrated. Aspects of themethod include the creation of a layer with the correct composition andthickness to facilitate polarity inversion and growth conditionsresulting in simultaneous growth of +c and −c face material at similarrates over the inversion layer and substrate while maintaining smoothgrowth surfaces. The versatility of this method allows for severalvertical polarity inversions through the repeated application of anappropriate inversion layer. Also, although the details focus onAl-based inversion layers, the patterning, which maintains the highestquality epitaxial surface by reducing reliance on etching getting rid ofall plasma etching steps, is comparable for use with other inversionlayers such as highly doped layers and Mg_(x)N_(y) layers. Thedemonstrations have been on samples patterned in stripes, but thetechnique can be used to form any device pattern required. This methodmay also be extended to SiC substrates. Additionally, a backside polishmay be employed to remove or thin the substrate for better thermalcontact.

The different faces of GaN-based material have opposing piezoelectricfields and induced surface charges. For device structures, this meansthat by having control over both N- to Ga- and Ga- to N-polarity, theinjection of holes or electrons can be controlled locally, leading toengineered charge inversion for lasers/LEDs and other novel electronicdevices. The technique is flexible enough to be used with any devicerequiring localized electron-hole densities and could be used as afoundation of CMOS technology. Devices of opposing polarities can begrown on the same wafer.

For optical devices, when patterned with an appropriate periodicity, thealternating polarities of GaN can be used in a quasi-phase-matchedoptical frequency converter to, for example, double the frequency oflaser light that passes through the device. Furthermore, by using a GaNsubstrate, the periodically patterned template can be grown bymetalorganic chemical vapor deposition (MOCVD) or molecular beam epitaxy(MBE) then very thick growth (>100 μm) for these type of devices can beattained by using HVPE growth. A polishing step may be added prior orafter HVPE growth to ensure a smooth surface. By selectively etching outthe N-polar regions by a simple wet-etch, high quality waveguides can beproduced. Additional polishing may be required for waveguide use.

Through the use of additional inversion and III-N epitaxial layers, anyhorizontal and vertical arrangement of N-polar and III-polar regions canbe formed. FIG. 1 shows a complex 3-D checkerboard pattern. This may bemade by, for example, depositing a checkerboard pattern of AlN on anN-polar substrate and depositing a first checkerboard layer of GaNthereon. The next layer may be formed by depositing the same pattern ofAl₂O₃ and the inverse pattern of AlN and growing another layer of GaN.The process is repeated, alternating the Al₂O₃ and AlN patterns. Similarmethods may be used to form other 3-D arrangements.

A potential advantage of the technique lies in the abruptness of theinterfaces between the both vertical and lateral polarity areas. This isillustrated in FIG. 6 where a cross-sectional scanning electronmicroscopy image of the lateral and vertical interface from a polarityswitch is shown on a GaN epilayer on sapphire. No gap is found betweenthe different polarity regions. Another potential advantage lays in thesmooth surfaces of both polarities. Generally −c face polar GaN ismorphologically rough, but the disclosed approach can result in a smoothsurface, which facilitates further processing and is necessary foroptical and electronic device performance. Furthermore, the growthconditions derived equalize the growth rates of the two polarities,resulting in a flat surface over the entire sample.

By establishing lateral periodic control of III-N polarity, lateralpolarity heterostructures become possible with unique electrical,optical, and structural properties. Using lateral periodic structures,GaN devices could find application in the areas of waveguides andfrequency converters. Current quasi-phase matching and 2^(nd) harmonicgeneration with periodically alternating structures relies on GaAs andLiNbO₃. The transparency windows for both of these are smaller andincluded in the window for GaN, and both materials have a lower thermalconductivity than GaN. Furthermore, with the ability to change polarityboth vertically and laterally, the potential of novel engineeredpolarity devices is opened. FIGS. 11A and 11B show schematics of some ofthe devices possible with the disclosed technique. FIG. 11A: Wavelengthconverter, FIG. 11B: waveguide, where the voids are left after etchingout N-polar material.

The following examples are given to illustrate specific applications.These specific examples are not intended to limit the scope of thedisclosure in this application.

EXAMPLE 1

Sapphire substrate—The samples used in this study were grown in a ThomasSwan MOCVD vertical showerhead reactor. MOCVD-grown templates consistingof 1.5-2 μm thick epilayers of N-polar GaN on sapphire were used assubstrates for a majority of this work, although several final sampleswere demonstrated on free-standing HVPE N-polar GaN substrates. Aselective growth approach was taken to create the patterned inversionlayer. In this case, the mask used consisted of stripes, but variousgeometries are possible. After patterning the substrate with standardphotolithography steps, a Si_(x)N_(y) layer was blanket deposited.Lift-off of the resist resulted in a Si_(x)N_(y) masked pattern forconfined epitaxial growth of the AlN inversion layer. This process hasbeen previously developed and used in selective growth studies of III-Nmaterials. The inversion layer was grown to a total thickness of 30 nmof AlN using trimethylaluminum and ammonia. Other conditions during thisgrowth included a V/III ratio over 30,000, chamber pressure of 60 Torr,and growth temperature between 690-710° C. Afterwards, the mask wasremoved by immersion in HF. At this point, the sample consists of AlNstripes on an N-polar GaN surface. The sample was then returned to theCVD chamber for a blanket growth of GaN material. The result is thatGa-polar GaN grows on top of the AlN inversion stripes and homoepitaxialN-polar GaN grows on the bare substrate simultaneously. For this growth,the V/III ratio was held at 3000, the chamber pressure was 150 Torr, andthe growth temperature was between 1030-1050° C. Nitrogen was added as acarrier gas to assist in equalizing the growth rate of the twopolarities (Mita et al., J. Cryst. Growth 311 (2009) 3044). The gas flowrates are as follows: 1.65 slm NH₃, 24.85 μmol TMG, 1.5 slm N₂, and 1.5slm H₂.

The morphology of the samples was characterized by scanning electronmicroscopy (SEM) in a LEO SUPRA 55 system. Structural properties of thefilms along with dislocation densities of each polarity wereinvestigated with electron channeling contrast imaging (ECCI), using aFEI Nova 600 NanoLab SEM and hkl Technology Nordlys electron backscatterdiffraction (EBSD) detector. This non-destructive characterizationmethod has been shown to pinpoint dislocations intersecting the samplesurface. Additionally, it can be used to distinguish between edge andscrew/mixed dislocations in both GaN and SiC materials (Picard et al.,Appl. Phys. Lett. 91 (2007) 094106; Picard et al., Appl. Phys. Lett. 90(2007) 234101). Structural characteristics were further determined bydark-field transmission electron microscopy of specimens extracted bythe Focused Ion Beam (FIB) lift out technique. Confirmation of N- vs.Ga-polar regions was determined by etching the samples in an alkalisolution of 4 M KOH at 35° C. for 10-40 minutes. N-polar material ismore chemically reactive than Ga-polar, hence, the N-polar material willetch with hexagonal faceting, whereas the Ga-polar is etch resistant andremains smooth (Zhuang et al., Materials Science and Engineering R 48(2005) 1).

The examples predominantly employed MOCVD-grown N-polar GaN templates onsapphire as substrates, which typically show hexagonal hillocksthroughout the surface. Using the process described above, alternatingstripes of Ga- and N-polar GaN were grown. In this mask set, the stripeshave 32 μm periodicity (16 μm wide stripes). An example of the resultsfrom this process is shown in FIGS. 4A and B. In SEM images the N-polarregions appear darker than the Ga-polar ones because they are moreconductive. Also, the structure of the substrate (hexagonal hillocks)remains visible underlying the alternating polarity structure. Even witha morphologically rich sample such as this, the boundary between thestripes appears abrupt and as straight as allowed by the underlyingstructure. A magnified view of the interface between Ga- and N-polarregions is shown in FIG. 4B. This image shows a difference in growthover the template's hexagonal hillocks. On the N-polar side, the growthpattern continues in the sharp, angular pattern established by thehexagonal hillocks. However, on the Ga-polar side, the hexagonal stepedges assume a more rounded, sloping appearance. The surface of eachstripe appears smooth, without any pitting or voids in the pattern. Forcomparison, a lateral polarity structure created by the same method, buton a bare sapphire wafer is shown in FIG. 5A. On the bare sapphire, theirregularities seen for samples grown on the N-polar templatesdisappear, leaving a smooth, regular pattern. From this comparison, alldeviations in the stripes in the samples grown on the N-polar templatesare attributed to morphological issues of the underlying layer. Oneadditional piece of information gleaned from growth on bare sapphireshows that the N-polar field is macroscopically rough, with hexagonalhillocks forming (outside of stripes in FIG. 5B), whereas the Ga-polarstripes are smooth. This shows that the conditions used to grow thelateral polarity structure are not optimal for smooth N-polar material.However, the simultaneous growth of N- and Ga-polar material at similargrowth rates serves to confine the N-polar growth (in between stripes),inhibiting hillock formation and producing a smooth surface. Previouswork in selective growth supports this, as it has shown that confinementof growth results in smoother surfaces (Picard et al., Appl. Phys. Lett.91 (2007) 094106). From these observations, growth on a flat substrateN-polar substrate, for example, on a HVPE substrate, should also resultin smooth N-polar stripes.

A cross-sectional SEM image of the interface between polarities is shownin FIG. 6. In this sample, the layer with a lateral variation inpolarity was grown to a thickness of 0.5 μm on a 2 μm N-polar epilayertemplate on sapphire (resulting in a total GaN thickness of 2.5 μm). Inthe image, the dark strip of the inversion layer is clearly visiblebetween the N-polar epilayer and inverted Ga-polar region. The directionof vertical inversion is marked by a white double-headed arrow. As seenin the cross-section, the surface of the lateral interface between thetwo polarities (highlighted with a black double-headed arrow) is quitesmooth, reflecting the similar growth rates achieved for both materials.Confirmation of polarity in GaN was readily achieved through etching thesamples in KOH. In all cases, the material grown over the inversionlayer was etch-resistant, while the N-polar field was quickly etched.

To further understand the effect of the inversion layer on the materialquality, a similar sample, but in this case consisting of 1.5 μm ofN-polar template GaN as well as 1.5 μm of alternating lateral polaritygrowth, was studied using ECCI. ECCI images of Ga- and N-polar regionstaken at the same magnification are shown in FIGS. 7A and 7B. Examplesof threading screw/mixed dislocations are indicated by black arrows,while areas with threading edge dislocations are circled. In thebottom-most circle in FIG. 7A, the threading edge dislocations can beseen to ring an area of darker contrast, which indicates a small anglegrain boundary in the material. The parallel lines running through theimages show the step flow of the material, examples of which are shownas dotted lines. In the Ga-polar material, portions of these lines waveand meander on the surface, while in the N-polar material, they areregular and straight. This difference in step flow is consistent withthe differences seen in the morphology over the hexagonal hillocksillustrated in FIG. 4B, where the N-polar growth maintains sharpcrystallographic steps and the Ga-polar growth has rounded edges. Inexamining the defectivity of the layers, the dislocation density islower in the N-polar material than in the Ga-polar material. Forthreading screw/mixed dislocation density the N-polar material has8.8×10⁸ cm⁻², while the Ga-polar material contains 1.2×10⁹ cm⁻². Thetotal dislocation density (TDD), covering both edge and screw-typedislocations, is 2.0×10⁹ cm⁻² for Ga-polar and 1.2×10⁹ cm⁻² for N-polar.These levels are consistent with those typically observed inheteroepitaxial films for these respective thicknesses on sapphire. Partof the observed difference in TDD between the two polar regions can beattributed to the differences in thicknesses between the two layers. Asthe N-polar growth is not interrupted by the inversion layer, it istwice as thick (3.0 μm) as the Ga-polar layer (1.5 μm). It is well knownthat the dislocation density in GaN is reduced as the heteroepitaxiallayers growth thicker (Mathis et al., Phys. Status Solidi A 179 (2000)125). However, the thickness difference (1.5 μm) does not entirelyaccount for the dislocation density difference. The additional increasein dislocation density in the Ga-polar material is likely due to theinsertion of the inversion layer.

Employing TEM allows a much more detailed study of the crystallographicstructure of the material. FIG. 8 shows a high resolution,cross-sectional TEM image of the intersection of N- and Ga-polar areas,similar to the cross-section in FIG. 3. From dark field TEM images usingthe g=0002 diffraction condition, it was determined that, although theN-polar material included inversion domain boundaries (IDB s, examplesindicated by black arrows) throughout the epilayer, suggesting thatlayer has yet to be optimized (Rouviere et al., Mater. Sci. Eng. B 43(1997) 161), the layer of Ga-polar GaN does not contain IDBs. Thisconfirms that the layer above the inversion layer has been completelyconverted to Ga-polar material, and is an improvement over the use ofMg-doping to produce polarity inversion. In addition, there is a strongIDB at the lateral interface between the Ga- and N-polar materials,which is indicated by a white arrow. The IDB at the lateral interfaceshows that the conversion between polarities is abrupt. Furthermore, incompliance with the larger dislocation density in the inverted Ga-polarmaterial discovered via ECCI, TEM uncovers additional dislocationsintroduced at the AlN/GaN interface. As has been seen in AlN/GaNsuperlattice structures, the AlN layer interferes with the propagationof threading dislocations from the preceding GaN layer. However, in thiscase, the AlN layer is also introducing new dislocations into subsequentlayers (Dadgar et al., Appl. Phys. Lett. 80 (2002) 3670; Amano et al.,MRS Internet J. Nitride Semicond. Res. 4S1 (1999) G10.1).

EXAMPLE 2

GaN substrate—The process was also employed on commercially available1×1 cm² HVPE N-polar GaN substrates. As the surfaces of these substrateshave been CMP-polished, the issues with initial morphology in the formof hexagonal hillocks seen on samples on the MOCVD-grown N-polartemplates should be eliminated. Indeed, SEM images of the lateralpolarity structure on the HVPE sample (FIG. 9) resemble the images oflateral polarity structures grown directly on sapphire substrates (FIG.5A). Although the surfaces of each polarity are still smooth, theinterface between the two polarities is not as abrupt. In addition, forthe N-polar regions, some regular faceting is seen. Both of these issuesare believed to arise from misalignment between the mask and thesubstrate crystallographic direction. It may be possible to resolve thisissue by improvements in lithographic procedures and betteridentification of crystallographic directions in the substrate crystal.

The slight difference in dislocation density between the polarities seenin structures grown on MOCVD-grown templates becomes much larger whenusing HVPE substrates. As seen in FIG. 10A, the dislocation density ofthe N-polar material is very low, on the order of 1×10⁷ cm⁻². This valueis comparable with expected values for high quality HVPE material; hencein the regrowth of homoepitaxial, homopolarity material, the extendeddefects of the underlying substrate continue, but additional defects atthis interface are not created. However, the dislocation density on the1.0 μm thick Ga-polar side is two orders of magnitude higher (2×10⁹cm⁻²). As seen in the TEM image (FIG. 8), the inverted Ga-polar GaNlayer is re-nucleating on the AlN IL, and as such is forming newdislocations at this interface which are not dependent on the underlyingN-polar layer. So, not only is the Ga-polar region much thinner than thetotal N-polar material, but new, substrate-independent defects are beingcreated at the nucleating AlN/GaN interface. As such, this level ofdefectivity may be inherent to the growth method. To address this issue,thicker growth, which is possible using HVPE substrates, should lead toadditional defect annihilation in the Ga-polar material, allowing it toapproach the N-polar quality. Alternatively, in confined growth usingoxide masks, it has been observed that aluminum precursors, in this casein the inversion layer, incorporate preferentially on the mask, and leadto a reduced growth rate in the confined region and, subsequently athinner than desired layer. This thinner layer then leads to a moredefective GaN top layer. Optimization of the AlN growth time may aid inreducing defects in the Ga-polar regions.

EXAMPLE 3

Templated substrate—A 2 μm thick MOCVD periodically patterned templatewas fabricated on a HVPE GaN substrate as described in Example 2. HVPEwas used to extend the growth of the template to 50 μm thick.Cross-sectional SEM of the sample (FIG. 12) showed the MOCVD/HVPEinterface to be sharp. Cross-sectional TEM of the sample (FIG. 13) showsthe continuation of the alternating polarity in the template through theHVPE growth, as evidenced by the sharp inversion domain boundariesbetween the regions. TEM also shows a noticeable reduction indislocation density in the Ga-polar regions with HVPE regrowth. Thisonly improves with distance from the MOCVD/HVPE interface. By growingthick material, the difference in dislocation density between the twopolar materials can be reduced. Photoluminescence mapping of the samplesurface also show evidence of the regular spacing of inversion domainboundaries. This example demonstrates the use of HVPE to extend thealternating polarity material, and can easily be extended to over 5 mmthick films. Although, in this case, the HVPE growth did not result in aperfectly smooth surface, as found in the template, optimization of thegrowth conditions may be performed. Otherwise, an optical or chemicalmechanical polish may be employed if necessary.

The advantage of using a GaN substrate for the periodically patternedtemplate layer and thick HVPE grown GaN epitaxial is that there isexcellent thermal expansion coefficient match between a GaN substrateand a GaN epitaxial layer so that the GaN epitaxial layer can be grownas a thick epitaxial layer without the GaN epitaxial layer cracking. TheHVPE epitaxial layer growth can be optimized so that the epitaxial layergrowth on the nitrogen-face and the epitaxial layer growth on thegallium face is substantially vertical with vertical sidewall and onlysmall change in the lateral dimension at the top surface of thenitrogen-face epitaxial layer from the lateral dimension of thenitrogen-face material in the template layer and only small change inthe lateral dimension at the top surface of the gallium face epitaxiallayer from the lateral dimension of the gallium-face material in thetemplate layer. For optical parametric oscillator applications, apreferred embodiment is to have the nitrogen-face epitaxial materiallayer laterally coincident with the gallium-face material with small orno separation between the HVPE grown gallium-face and nitrogen facematerial layer. For the case that light is transmitted perpendicular tothe nitrogen-face and gallium face growth direction, the separationbetween the nitrogen face and gallium face material layers should beless than the wavelength of the light. The HVPE grown epitaxial layercan be optimized for optical transmission perpendicular to the growthdirection of the gallium face and nitrogen face material. In particular,the epitaxial layer can be optimized to have low impurities that canabsorb light or diffract light. The epitaxial growth method can beoptimized to have low extended defects and low material cracking thatcan absorb or diffract light transmission perpendicular to the growthdirection of the nitrogen and gallium faces.

EXAMPLE 4

Ga-Polar GaN substrate—A 2 μm thick Ga-polar GaN epilayer on sapphirewas used as a substrate. Atomic layer deposition (ALD) was used todeposit a 150 Å thick layer of Al₂O₃ over the entire substrate. Thislayer was patterned using standard photolithographic methods, and theopen areas were etched from the substrate using a hydrofluoric acidetchant (10 s, 35° C., 1:10 H₂0:HF). The sample was loaded into a MOCVDreactor, and annealed under ammonia and hydrogen (2 slm and 4.2 slm,respectively) above 1070° C. for 15 minutes. After annealing, a lowtemperature GaN layer was grown at 660° C. The temperature was thenramped up to a growth temperature of 1050° C., and left to anneal for 25minutes. After which, GaN was grown under conditions conducive toN-polar growth (0.5 slm ammonia, 22.2 μmol/min TMG, 4.0 slm nitrogen,2.2 slm hydrogen). As observed in SEM images, the N-polar material hasfully coalesced over the ALD Al₂O₃ layer, demonstrating that this is aviable technique for selectively controlling GaN polarity on Ga-polarsubstrates. In this case, there was a smooth surface over the entirewafer (FIG. 14).

Obviously, many modifications and variations are possible in light ofthe above teachings. It is therefore to be understood that the claimedsubject matter may be practiced otherwise than as specificallydescribed. Any reference to claim elements in the singular, e.g., usingthe articles “a,” “an,” “the,” or “said” is not construed as limitingthe element to the singular.

What is claimed is:
 1. A method comprising: providing a sapphire,silicon, or silicon carbide substrate having an epitaxial layer ofN-polar GaN; depositing a mask layer on the epitaxial layer; removing apattern from the mask layer to produce open areas and masked areas;depositing an AlN inversion layer on the open areas; removing anyremaining mask layer from the substrate; and depositing a III-Nepitaxial layer on the N-polar GaN epitaxial layer and the AlN layer tosimultaneously produce N-polar material on the N-polar GaN epitaxiallayer and III-polar material on the AlN layer.
 2. The method of claim 1;wherein depositing the mask layer comprises: depositing a layer oflift-off material on the N-polar GaN epitaxial layer; lithographicallyprocessing the lift-off material to produce the open areas and themasked areas; and depositing a mask material on the entire substrate;and wherein removing the pattern comprises: removing any remaininglift-off material and any mask material disposed thereon from thesubstrate.
 3. The method of claim 1, wherein the III-N epitaxial layercomprises GaN, InGaN, AlGaN, or AlInGaN.
 4. A method comprising:providing a III-polar bulk III-N substrate or a sapphire substratehaving an epitaxial layer of III-polar III-N material; depositing aninversion layer on the substrate; and depositing a III-N epitaxial layeron the inversion layer to produce N-polar material on the inversionlayer.
 5. The method of claim 4; wherein the inversion layer isdeposited in a pattern leaving exposed portions of the substrate; andwherein depositing the III-N epitaxial layer simultaneously producesN-polar material on the inversion layer and III-polar material on theexposed III-polar III-N substrate.
 6. The method of claim 4, wherein theinversion layer comprises Al₂O₃, Mg-doped GaN, or Mg_(x)N_(y).
 7. Themethod of claim 4, wherein the III-polar bulk III-N substrate is a GaNsubstrate.
 8. The method of claim 4, wherein the III-N epitaxial layercomprises GaN, InGaN, AlGaN, or AlInGaN.
 9. The method of claim 4,wherein the III-N epitaxial layer is deposited by hydride vapor phaseepitaxy.
 10. A composition comprising: a bulk III-N substrate; aninversion layer on portions of an N-polar or III-polar surface of thesubstrate; and a III-N epitaxial layer on the inversion layer; whereinthe III-N epitaxial layer is of the opposite polarity of the surface ofthe substrate.
 11. The composition of claim 10, wherein the bulk III-Nsubstrate is N-polar GaN and the inversion layer comprises AlN.
 12. Thecomposition of claim 10, wherein the bulk III-N substrate is Ga-polarGaN and the inversion layer comprises Al₂O₃.
 13. The composition ofclaim 10, wherein the III-N epitaxial layer comprises GaN, InGaN, AlGaN,or AlInGaN.
 14. The composition of claim 10, wherein the III-N epitaxiallayer is 0.2-50 microns thick.
 15. The composition of claim 10, whereinthe III-N epitaxial layer comprises: a first layer of III-N materialgrown by metalorganic chemical vapor deposition or molecular beamepitaxy; and a second layer of III-N material grown by hydride vaporphase epitaxy that is up to 5 mm thick.
 16. The composition of claim 10;wherein the III-N epitaxial layer extends to cover portions of thesubstrate not having the inversion layer thereon; and wherein theportion of the III-N epitaxial layer that covers portions of thesubstrate not having the inversion layer thereon is of the same polarityof the surface of the substrate.
 17. The composition of claim 16,wherein the III-N epitaxial layer comprises a region of laterallyalternating bands of III-polar and N-polar material.
 18. Aquasi-phase-matched optical frequency converter comprising thecomposition of claim
 17. 19. The composition of claim 16, furthercomprising: one or more additional III-N epitaxial layers on the III-Nepitaxial layer; wherein at least one of the additional III-N epitaxiallayers comprises a region of opposite polarity of an adjacent III-Nepitaxial layer.